1. Field of the Invention
The field of the invention relates generally to power production from distributed DC power sources, and more particularly to management of distributed DC power sources in series installations.
2. Related Arts
The recent increased interest in renewable energy has led to increased research in systems for distributed generation of energy, such as photovoltaic cells (PV), fuel cells, batteries (e.g., for hybrid cars), etc. Various topologies have been proposed for connecting these power sources to the load, taking into consideration various parameters, such as voltage/current requirements, operating conditions, reliability, safety, costs, etc. For example, most of these sources provide low voltage output (normally a few volts for one cell, or a few tens of volts for serially connected cells), so that many of them need to be connected serially to achieve the required operating voltage. Conversely, a serial connection may fail to provide the required current, so that several strings of serial connections may need to be connected in parallel to provide the required current.
It is also known that power generation from each of these sources depends on manufacturing, operating, and environmental conditions. For example, various inconsistencies in manufacturing may cause two identical sources to provide different output characteristics. Similarly, two identical sources may react differently to operating and/or environmental conditions, such as load, temperature, etc. In practical installations, different source may also experience different environmental conditions, e.g., in solar power installations some panels may be exposed to full sun, while others be shaded, thereby delivering different power output. In a multiple-battery installation, some of the batteries may age differently, thereby delivering different power output. While these problems and the solutions provided by the subject invention are applicable to any distributed power system, the following discussion turns to solar energy so as to provide better understanding by way of a concrete example.
A conventional installation of solar power system 10 is illustrated in FIG. 1. Since the voltage provided by each individual solar panel 101 is low, several panels are connected in series to form a string of panels 103. For a large installation, when higher current is required, several strings 103 may be connected in parallel to form the overall system 10. The solar panels are mounted outdoors, and their leads are connected to a maximum power point tracking (MPPT) module 107 and then to an inverter 104. The MPPT 107 is typically implemented as part of the inverter 104. The harvested power from the DC sources is delivered to the inverter 104, which converts the fluctuating direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at 60 Hz, or 220V at 50 Hz (It is interesting to note the even in the US many inverters produce 220V, which is then split into two 110V feeds in the electric box). The AC current from the inverter 104 may then be used for operating electric appliances or fed to the power grid. Alternatively, if the installation is not tied to the grid, the power extracted from the inverter may be directed to a conversion and charge/discharge circuit to store the excess power created as charge in batteries. In case of a battery-tied application, the inversion stage might be skipped altogether, and the DC output of the MPPT stage 107 may be fed into the charge/discharge circuit.
As noted above, each solar panel 101 supplies relatively very low voltage and current. The problem facing the solar array designer is to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage requires very high currents, which cause large conduction losses on the order of the second power of the current (I2). Furthermore, a power inverter, such as the inverter 104, which is used to convert DC current to AC current, is most efficient when its input voltage is slightly higher than its output RMS voltage multiplied by the square root of 2. Hence, in many applications, the power sources, such as the solar panels 101, are combined in order to reach the correct voltage or current. The most common method connects the power sources in series in order to reach the desirable voltage and in parallel in order to reach the desirable current, as shown in FIG. 1. A large number of the panels 101 are connected into a string 103 and the strings 103 are connected in parallel to the power inverter 104. The panels 101 are connected in series in order to reach the minimal voltage required for the inverter. Multiple strings 103 are connected in parallel into an array to supply higher current, so as to enable higher power output.
While this configuration is advantageous in terms of cost and architecture simplicity, several drawbacks have been identified in the literature for such architecture. One recognized drawback is inefficiencies cause by non-optimal power draw from each individual panel, as explained below. As explained above, the output of the DC power sources is influenced by many conditions. Therefore, to maximize the power draw from each source, one needs to draw the combination of voltage and current that provides the peak power for the currently prevailing conditions. As conditions change, the combination of voltage and current draw may need to be changed as well.
FIG. 2 illustrates one serial string of DC sources, e.g., solar panels 201 a-201 d, connected to MPPT circuit 207 and inverter 204. The current versus voltage (IV) characteristics plotted (210a-210d) to the left of each DC source 201. For each DC source 201, the current decreases as the output voltage increases. At some voltage value the current goes to zero, and in some applications may assume a negative value, meaning that the source becomes a sink. Bypass diodes are used to prevent the source from becoming a sink. The power output of each source 201, which is equal to the product of current and voltage (P=I*V), varies depending on the voltage drawn from the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point. The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources.
In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of the system 10 using the MPPT module 107. The MPPT module 107 receives the current extracted from all of the solar panels together and tracks the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. The MPPT module 107 maintains a current that yields the maximum average power from the overall system 10.
Maximum power point tracking techniques are reviewed in: “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques” by T. Esram & P. L. Chapman, IEEE Transactions on Energy Conversion (Accepted for future publication, Volume PP, Issue 99, 2006 Page(s): 1-1, Digital Object Identifier 10.1109/TEC.2006.874230), the entire content of which is incorporated herein by this reference.
However, since the sources 201a-201d are connected in series to a single MPPT 207, the MPPT must select a single point, which would be somewhat of an average of the MPP of the serially connected sources. In practice, it is very likely that the MPPT would operate at an I-V point that is optimum to only a few or none of the sources. In the example of FIG. 2, the selected point is the maximum power point for source 201b, but is off the maximum power point for sources 201a, 201c and 201d. Consequently, the arrangement is not operated at best achievable efficiency.
Turning back to the example of a solar system 10 of FIG. 1, fixing a predetermined constant output voltage from the strings 103 may cause the solar panels to supply lower output power than otherwise possible. Further, each string carries a single current that is passed through all of the solar panels along the string. If the solar panels are mismatched due to manufacturing differences, aging or if they malfunction or are placed under different shading conditions, the current, voltage and power output of each panel will be different. Forcing a single current through all of the panels of the string causes the individual panels to work at a non-optimal power point and can also cause panels which are highly mismatched to generate “hot spots” due to the high current flowing through them. Due to these and other drawbacks of conventional centralized methods, the solar panels have to be matched properly. In some cases external diodes are used to bypass the panels that are highly mismatched. In conventional multiple string configurations all strings have to be composed of exactly the same number of solar panels and the panels are selected of the same model and must be install at exactly the same spatial orientation, being exposed to the same sunlight conditions at all times. This is difficult to achieve and can be very costly.
Various different topologies have been proposed in order to overcome the above deficiencies of the serial installation. For example, some have proposed to have inverters coupled to each DC source, and connect all of the inverters in parallel. Others have proposed to have DC/DC converter connected to each DC source, and to connect all of the converters serially or in parallel to a central inverter. Among the DC/DC converters proposed for use with the DC sources are boost converter, buck converter, buck-boost converter, or a Cuk converter. It has also been proposed to incorporate MPPT into each DC power source, e.g., into each solar panel, and connect the panels serially.
For further discussion of the above issues relating to distributed power sources and solar panels, the reader is highly encouraged to review the following literature, which may or may not be prior art.    Cascade DC-DC Converter Connection of Photovoltaic Modules, G. R. Walker and P. C. Sernia, Power Electronics Specialists Conference, 2002 (PESC 02), Vol. 1 IEEE, Cairns, Australia, pp. 24-29.    Topology for Decentralized Solar Energy Inverters with a Low Voltage AC-Bus, Bjorn Lindgren.    Integrated Photovoltaic Maximum Power Point Tracking Converter, Johan H. R. Enslin et al., IEEE Transactions on Industrial Electronics, Vol. 44, No. 6, December 1997.    A New Distributed Converter Interface for PV Panels, R. Alonso et al., 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain.    Intelligent PV Module for Grid-Connected PV Systems, Eduardo Roman, et al., IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006. Also in Spanish patent application ES2249147.    A Modular Fuel Cell, Modular DC-DC Converter Concept for High Performance and Enhanced Reliability, L. Palma and P. Enjeti, Power Electronics Specialists Conference, 2007, PESC 2007, IEEE Volume, Issue, 17-21 Jun. 2007 Page(s): 2633-2638. Digital Object Identifier 10.1109/PESC.2007.4342432.    Experimental Results of Intelligent PV Module for Grid-Connected PV Systems, R. Alonso et al., Twenty first European Photovoltaic Solar Energy Conference, Proceedings of the International Conference held in Dresden, Germany, 4-8 Sep. 2006.    Cascaded DC-DC Converter Connection of Photovoltaic Modules, G. R. Walker and P. C. Sernia, IEEE Transactions on Power Electronics, Vol. 19, No. 4, July 2004.    Cost Effectiveness of Shadow Tolerant Photovoltaic Systems, Quaschning, V.; Piske, R.; Hanitsch, R., Euronsun 96, Freiburg, Sep. 16-19, 1996.    Evaluation Test results of a New Distributed MPPT Converter, R. Orduz and M. A. Egido, 22nd European Photovoltaic Solar Energy Conference, 3-7 Sep. 2007, Milan, Italy.    Energy Integrated Management System for PV Applications, S. Uriarte et al., 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain.    U.S. Published Application 2006/0185727
As noted in some of the above cited works, integrating inverters into the individual cells has many drawbacks, including high costs, low safety (especially in solar installations), and reliability. Therefore, serial connection is still preferred, especially for solar panel installations. The proposals for including DC-DC converters and MPPT into the individual sources, and then connect their outputs serially to an inverter are attractive. However, incorporating MPPT into each panel is still problematic in serial application, as each MPPT would attempt to drive its source at different current, while in a serial connection the same current must flow through all of the panels. Moreover, it is unclear what type of DC-DC converter would provide the best results and how to incorporate an MPPT into such an arrangement. Therefore, solutions are still needed for an effective topology for connecting multiple DC power sources to the load, i.e., power grid, power storage bank, etc.
As already mentioned above, various environmental and operational conditions impact the power output of DC power sources. In the case of solar panels, solar radiance, ambient temperature, and shading, whether from near objects such as trees or far objects such as clouds, impact the power extracted from each solar panel. Depending on the number and type of panels used, the extracted power may vary widely in the voltage and current. Owners and even professional installers find it difficult to verify the correct operation of the solar system. With time, many other factors, such as aging, dust and dirt collection and module degradation affect the performance of the solar array.
The sensitivity of photovoltaic panels to external conditions is even more profound when concentrated photovoltaics (CPV) are used. In such installations, the sun radiation is concentrated by use of lenses or mirrors onto small cells. These cells may be much more efficient then typical PV cells and use a technology knows as double- or triple-junction, in which a number of p-n junctions are constructed one on top of the other—each junction converts light from a certain part of the spectrum and allows the rest to pass-through to the next junction. Thus, these cells are much more efficient (with peak efficiencies of over 40%). Since these cells are expensive, they are usually used in CPV applications which call for smaller cells. However, the power output of CPV installations now depends upon fluctuations in the intensity of different parts of the spectrum of the sun (and not only the total intensity), and imperfections or distortions in the lenses or mirrors used. Thus, having a single MPPT for many panels will lead to significant power loss, and great benefits are realized from using a panel- (or cell-) level MPPT as described in aspects of the present invention.
Another field in which traditional photovoltaic installations face many problems is the developing market of building-integrated photovoltaics (BIPV). In BIPV installations, the panels are integrated into buildings during construction—either as roof panels or as structural or additional elements in the walls and windows. Thus, BIPV installations suffer greatly from local partial shading due to the existence of other structural elements in the vicinity of the panels. Moreover, the panels are naturally positioned on many different facets of the building, and therefore the lighting conditions each panel experiences may vary greatly. Since in traditional solutions the panels are stringed together to a joint MPPT, much power is lost. A solution that could harvest more power would obviously be very beneficial in installations of this type.
Yet another problem with traditional installations is the poor energy utilization in cases of low sun-light. Most inverters require a certain minimal voltage (typically between 150V to 350V) in order to start functioning. If there is low light, the aggregated voltage from the panels may not reach this minimal value, and the power is thus lost. A solution that could boost the voltage of panels suffering from low light, would therefore allow for the produced energy to be harvested.
During installation of a solar array according to the conventional configurations 10, the installer can verify the correctness of the installation and performance of the solar array by using test equipment to check the current-voltage characteristics of each panel, each string and the entire array. In practice, however, individual panels and strings are generally either not tested at all or tested only prior to connection. This happens because current measurement is done by either a series connection to the solar array or a series resistor in the array which is typically not convenient. Instead, only high-level pass/fail testing of the overall installation is performed.
After the initial testing of the installation, the solar array is connected to inverter 104 which optionally includes a monitoring module which monitors performance of the entire array. The performance information gathered from monitoring within the inverter 104 includes integrated power output of the array and the power production rate, but the information lacks any fine details about the functioning of individual solar panels. Therefore, the performance information provided by monitoring at the inverter 104 is usually not sufficient to understand if power loss is due to environmental conditions, from malfunctions or from poor installation or maintenance of the solar array. Furthermore, integrated information does not pinpoint which of solar panels 101 is responsible for a detected power loss.
In view of the above, a newly proposed topology for connecting multiple DC power sources to the load should also lend itself to easy testing and operational verification during and after installation.